Weird error when I create a blendshape
It appears, that there is a relative deformation applied to the mesh after the shapekey.

This is likely to be caused by a deforming modifier, like an armature modifier. Modifiers are applied after shapekeys. To properly view your shapekey in object mode you can 


turn of (the armature) modifier
reset the armature to rest pose with the Rest Position button

The armature will be unposable as long as this button is activated.
reset the transformations of the armature, by


showing all armature layers
selecting all bones in pose mode (A)
Removing keyframes which would prevent the bones from being reset.
clearing their transformations with &#9095; AltG (Translation), &#9095; AltR (Rotation) and &#9095; AltS (scale).